Saponified (solution type) rosin sizing agents have been used for internal addition in a process for manufacturing paper under acidic conditions together with the use of aluminum sulfate. However, such a sizing agent exerts a poor sizing effect at a low addition level. Moreover, the sizing effect is further reduced due to an increase in water temperature accompanying the recent closed drainage system or within a range around neutral pH. In order to overcome the above-mentioned disadvantages of the saponified rosin sizing agents, emulsion type rosin sizing agents have been developed. Under the present conditions, however, these emulsion type robin sizing agents are also unsatisfactory because of the poor sizing effects at a low addition level or within a range around neutral pH.
In order to compensate for the disadvantage of these rosin-series sizing agents, saponified alkenylsuccinic acids have recently been used. These are obtained by saponifying alkenylsuccinic acids with alkalis, as a sizing agent exhibiting an excellent sizing effect at a low addition level (JP-A-58-214598), (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, with these sizing agents the sizing effects thereof are deteriorated in papermaking at a high temperature or within a range around neutral pH, similar to the saponified rosin sizing agents.
A method wherein an alkenylsuccinic anhydride blended with an emulsifier is emulsified with a solution of cationic starch or water at a low concentration (about 0.5 to 3%) and used for neutral papermaking (U.S. Pat. No. 3,821,069) is known.
The reaction responsible for alkenylsuccinic anhydride sizing is as follows. Sizing is achieved by the anhydride group in the alkenylsuccinic anhydride molecule esterifying a fiber hydroxyl group of cellulose, i.e., the main component of pulp, and thus, introducing the substituent hydrophobic alkenyl group onto the fiber surface. Because of the higher reactivity of alkenylsuccinic anhydride, it is known to form an ester bond with the hydroxyl group of cellulose. However, the alkenylsuccinic anhydride is highly reactive with water too. When the alkenylsuccinic anhydride is preliminarily emulsified and dispersed in water, therefore, it reacts with water within a short period of time. As a result, an acid anhydride group is converted into a carboxyl group, which causes a loss in function as a neutral sizing agent. During the process of the conversion of the acid anhydride group into the carboxyl group due to the reaction with water, the emulsion system is changed displaying aggregation, precipitation and separation. Therefore, an emulsion sizing agent comprising an alkenylsuccinic anhydride for neutral papermaking, in the form of an aqueous dispersion can be stored for only several hours. Accordingly, it cannot be supplied in the form of an emulsion product with a high concentration. This characteristic requires that it be emulsified and dispersed by using an emulsifier immediately before the papermaking.
It is also known that an alkenylsuccinic anhydride shows a lower esterification reactivity of he acid anhydride group under acidic conditions than under neutral or weakly alkaline conditions. When an emulsion of an alkenylsuccinic anhydride is used under acidic papermaking conditions (in particular, at a pH value of 5.0 or below), the sizing effect appears slowly, and only a poor sizing effect can be achieved immediately after the papermaking. If an alkenylsuccinic anhydride could be hydrolyzed into the corresponding alkenylsuccinic acid and then emulsified, the above-mentioned problem of changes (i.e., aggregation, precipitation, separation) in the emulsion system might be solved, and the alkenylsuccinic acid would rapidly react with aluminum sulfate even in an acidic region to thereby exert an excellent sizing effect immediately after the papermaking. However an alkenylsuccinic acid can hardly be emulsified due to its highly hydrophilic nature. It is therefore difficult to obtain an emulsion which has a higher concentration than the conventional emulsion type rosin sizing agents and which can remain stable over a long period.
It has been known to use a wax as a water repellent agent. In sizing agents for papermaking, emulsions of paraffin wax and montan wax have been used to increase water repellency and decrease cost. In recent years, emulsion sizing agents wherein an alkenylsuccinic acid is blended with paraffin wax or maleic wax (JP-A-62-70479) have been proposed. However these sizing agents are disadvantageous in that the storage stabilities and sizing effects of the same are limited or a large amount of an emulsifier is required.